The present invention relates to a Coriolis sensor interface, and more particularly to a method of biasing and sensing the resonator for a Coriolis sensor.
Resonator gyroscopes are used in many applications including, but not limited to, communications, control and navigation systems for both space and land applications. These highly specialized applications require high performance and cost effective gyroscopes.
In a vibratory gyroscope, the Coriolis effect induces energy transfer from the driver input vibratory mode to another mode which is sensed or output during rotation of the gyroscope. Examples of vibratory gyroscopes include a xe2x80x9ccloverleafxe2x80x9d gyroscope and a hemispherical resonator gyroscope.
The cloverleaf gyroscope is explained and described in a technical paper entitled xe2x80x9cSilicon Bulk Micro-machined Vibratory Gyroscopexe2x80x9d presented in June, 1996 at the Solid State Sensors and Actuator Workshop in Hilton Head, S.C.
The micro-machined electromechanical gyroscope has a resonator having a xe2x80x9ccloverleafxe2x80x9d structure consisting of a rim, four silicon leaves, and four soft supports, or cantilevers, made from a single. crystal silicon. The four supports provide mechanical support and restoring force for the harmonic motion of the structure. A post is rigidly attached to the center of the resonator, in a plane perpendicular to the plane of the silicon leaves, and to a quartz base plate spaced apart from the silicon leaves. The quartz base plate has a pattern of electrodes that coincides with the cloverleaf pattern of the silicon leaves. The electrodes include two drive electrodes and two sense electrodes. The post provides large Coriolis coupling that transfers energy between the two orthogonal rocking modes.
The control electronics associated with the cloverleaf micro-gyroscope includes an electrically grounded resonator. A resonator bias voltage is applied to the sense and control electrodes. The bias voltage is limited to allow sufficient dynamic range for each amplifier which is limited by supply voltages.
Simple transresistance sense amplifiers are referenced to the resonator bias voltage. The sum of the bias voltage and signal dynamic range is limited by the amplifier power supply voltage.
The HRG is described and explained in a technical paper entitled; xe2x80x9cA Micromachined Vibratory Ring Gyroscopexe2x80x9d, by M. W. Putty, and K. Najafi, Solid State Sensor and Actuator Workshop, Hilton Head, S.C., June, 1994.
The HRG is made of quartz and has a shell resonator design. Three hemispherical shells are used as vibratory elements to detect rotation about three mutually orthogonal axes. It is immune to external vibration and is capable of standing high g shocks.
The HRG has an independently biased resonator. However, it uses a complex bootstrap buffer for capacitive sensing. This method of sensing does not produce a voltage proportional to the resonator velocity, resulting in more complex and expensive control electronics to derive signals in phase with velocity from position. The HRG has four control loops which use multiple demodulators, modulators and a phase lock loop.
All of the signal processing for the cloverleaf gyroscope and the HRG is done with analog components. The selected components have a high radiation tolerance for space.
In general, the signals from the sense electrodes are summed to remove the differential signal between them and the response of the sense resonance from the feedback loop. On the other hand, the sense circuit subtracts the signals from the sense electrodes to remove the common-mode drive signal.
Micro-gyroscopes are subject to electrical interference that degrades performance with regard to drift and scale factor stability. Micro-gyroscopes often operate the drive and sense signals at the same frequency to allow for simple electronic circuits. However, the use of a common frequency for both functions allows the relatively powerful drive signal to inadvertently electrically couple to the relatively weak sense signal.
Typically, prior art micro-gyroscopes are open loop and untuned. If the drive frequency is tuned closely to a high Q sense axis resonance, large mechanical gain and low sensitivity to sensor noise is possible. High Q also results in low rate drift.
However, close tuning leads to large uncertainty in the gain and phase of the open-loop response. Phase variations lead to added rate drift errors due to quadrature signal pickup and the gain variations lead to rate scale factor errors. Operating the open-loop micro-gyroscope in a closely tuned manner results in higher scale factor error, higher rate errors due to mechanical phase shifts, and slower response with sensitive lightly damped resonances. Additionally, the response time of the open-loop micro-gyroscope is proportional to the damping time constant, Q, of the sense resonance. To reduce rate drift, very long natural damping time constants are required, slowing the response time.
If the drive frequency is tuned closely to a high Q sense axis resonance, a force-to-rebalance method that incorporates complex demodulators and modulators in multiple re-balance loops is necessary. The modulators and demodulators provide coherent feedback only for signals modulating the drive frequency, and therefore do not provide active damping of independent sense resonance vibrations. These vibrations, if not exactly matched to the drive frequency, are not actively damped resulting in false rate signals or noise.
Noise and drift in the electronic circuit limit micro-gyroscope performance. Therefore, prior art micro-gyroscopes perform poorly and are unreliable in sensitive space applications.
Previous open loop operation is intentionally split between two rocking mode frequencies. The rocking mode axes tend to align with the spring axes, electrode sense axes and electrode control axes. Closed loop control enables close tuning of the rocking modes. However, residual imbalances result in non-alignment of rocking mode axes with electrode axes. This produces a large quadrature error signal and second harmonics on the output axis sensor which limits the amount of amplification and closed loop gain that can be applied. The large quadrature error signal also causes false rate indications due to phase errors in the demodulation. The lack of tuning of the two modes due to mismatch of the spring reduces the sensor mechanical gain and increases rate noise.
The present invention is a common electrical interface and control method for both the silicon cloverleaf micro-gyroscope and the quartz HRG wherein the vibrating resonator is excited with a dc bias voltage and the rate of change in electrode capacitance is directly sensed by transimpedance amplifiers. The result is capacitive motion sensor outputs for the drive and output axis have lower noise, larger dynamic range and are proportional to the resonator velocity.
The velocity sensing permits simple agc drive axis control and direct wideband control of the output axis. Wideband control provides robust damping and control of the output axis resonance reducing sensitivity to external disturbances.
In the present invention, mixed analog and digital components in a common control loop will have reduced cost integrated circuit implementation. The cloverleaf micro-gyroscope and the HRG can share the same control electronics. The result is a reduction in the cost of development of low-cost space gyroscopes for a wide range of performance.
It is an object of the present invention to maximize the available bias voltage and maximize the available dynamic signal range for the sense and control functions. It is another object of the present invention to increase the rate scale factor, rate resolution and dynamic range of the micro-gyroscope. It is a further object of the present invention to simplify the control electronics.
Other objects and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims.